Sonic direction finding



l n 17, 1 A. A. STUART, JR

some DIRECTION FINDING Filed May 15, 1941 2 Sheets-Sheet 1 ffi w 'huart '2 V ATTORNEY June 17, 1947. A. A. STUART, JR 7 2,422,446

I SONIC DIRECTION FINDING Filed May 15, 1941 2 Shets-Sheet 2 V INVENTOR. A lfredA.Sluar- J1"; BY wrm ATTORNEYS.

Patented June 17, 1947 some pmnc'rlon'rnvnmo Alfred a. smart, Jr., Hasbrouck Heights, N. 1.,

assignor to Bendix Bendix, N. J., a corporation of Delaware Application May 15, 1941, Serial No. 393,685

11 Claims. 1 This invention relates to sonic direction finding devices and more particularly to devices responding to and adapted for ascertaining the direction of a source of sound, whereby a device will be automatically guided or directed toward, or in the direction of, the sound source.

An object of the present invention is to provide novel means for indicating the'direction of sound waves emanating from a desired source.

Another object is to provide a novel automatic remote indicator for indicating the direction of a source of sound in accordance with the sound waves emanating therefrom.

A further object of the invention is to provide a novel apparatus comprising directional and non-directional microphones adapted for intercepting sound waves emanating from a given source of sound whereby, upon a displacement of the directional microphone from a predetermined relationshipwith respect to the emanating sound waves, the latter microphone is caused to be turned bodily to its null position to thus continuously-point to the direction of the sourcefrom which the sound waves are radiating.

Another object is tq provide a novel apparatus comprising a non-directional microphone and a plurality of directional microphones so arranged as to be adapted to intercept sound waves radiating from a given source, whereby the apparatus continuously indicates the direction of the source from which the sound waves radiate.

A further object of the invention is to provide the novel combination of a rotatable directional microphone and a non-directional microphone adapted for intercepting sound waves radiating from a given source, and an induction motor whereby the latter is energized in accordance with the output of the microphones to rotate the directional microphone to its null position after a relative movement between the directional microphone and the source, thus continuuosly indicating the direction or the source oi the sound waves intercepted by the microphones.

Still another object is to provide a novel apparatus comprising a plurality of stationary directional microphones and a non-directional microphone, and a multi-phase induction motor the phases oi which are energized in accordance with the output of th microphones so as to operate an indicator tothereby indicate the direction of the source of sound.

Another and equally important object of the present invention is to provide novel means for automatically directing an object towards a desired source of sound.

Aviation Corporation,

A further object of the invention is to provide a novel automatic target-seeking diriglble bomb controlled by mechanism responsive to sound.

A still further object of the invention is to provide an automatic glider type device equipped with novel sound responding apparatus for directing or steering the glider in the direction of a desired source of sound.

A still further object 'of the invention is to provide a novel glider device having control surfaces thereon with a novel sonic responsive apparatus the output of which is utilized to control the glider surfaces thereby automatically directing the glider to a source of sound.

An additional object of the present invention is to provide a target seeking dirigible bomb in the form of a glider having a rudder and elevator control surfaces with a novel sonic responsive apparatus comprising a pair of directional mi-' crophones and a nonvdirectional microphone, whereby the rudder and.- elevator surfaces are controlled in response to the output of the microphones so that the target seeking dirigible bomb is automatically directed toward a desired target which may constitute a source of sound such as that defined by the propeller or engine of an enemy bomber while in flight. I

The above and other objects and novel features of the invention will more fully appear from the detailed description which follows when the same is read in connection with the accompanying drawings. It is to be expressly understood, however, that the drawings are for the purposes of illustration only and are not intended as a dennition of the limits of the invention, reference for the latter purpose being had primarily to the appended claims.

In the drawings, wherein like reference characters refer to like parts throughout the several VIEWS, a

Fig. l is a schematic representation of one form of the novel arrangement of the microphones in accordance with the present invention,

Fig. 2 is a schematic representation of a second form of the present invention embodying. the use of a plurality of directional microphones and a non-directional microphone in combination with indicating means for indicating the direction of a source of sound,

Fig. 3 is a graphic representation or pattern illustrating the output characteristics of the directional microphone when it is rotated 360 in azimuth, and

Fig. 4 is a diagrammatic and partially schematic representation of. a novel target seeking dirigible bomb equipped with the novel means of the present invention.

' The present invention contemplates the use or sonic responsive devices the-voltage output of which is dependent upon the angular displace 'ment or the devices wlthrespect toa source of sound, and which is utilized to either operatean indicator to thereby indicate the direction of a "sound source, or whlchcmay be converted into. driving power for automatically driving or steering aprojectile having control surfaces thereon to the source or sound. The-sonic responsive be of the ribbon or velocity type.

type microphone. having a magnet H between the polesoi" which is supported a ribbon l2 by I means'of suitable means such as Bakelite i3. Theribbon 12 is connectedby wayo! leads I4 to a suitable and conventional vacuum tube 8411? plifler to, the output of which is, in turn, connected by way'of leads I 6 toone phase of a twodevices may comprise either a rotatably molmt- I ed directional microphonetogether with a nondirectional microphone, the. output voltage of I both of the microphones'being amplified to energize an induction motonwherebythe latter op- I crates to rotate the directional microphone to its null position, or a plurality of directional continuously indicating the direction of the source of sound. However, an equally important j application of the above principle is contem plated 'in that the output of the microphones so i a device having control surfaces thereontoward may be ultimately utilized to automatically direct the source otsound; Thedevice having the concally'cont'rolled by the ultimate microphone output maybe another airplanaglider, target seeking dirigible bomb, etc.

A particular and desirable use of the abovedescribed automatic apparatus is for purposes of warfare. For example, in defensive warfare, a bomb built in the form of a glider having both elevator and-rudder surfaces may be carried by "an aircraft flying at an altitude higher than the altitude flown by enemy bombers and then' dropped in thegeneral' direction of the enemy trol surfaces thereon and adapted to be 'automatiis to be noted that the glider device is not Drovided with any motivating power of its own and thus will tend to fall-toward the earth only and will deviate from this path in response to the sound waves emanating from the enemy target at an altitude lower than the altitude of the craft from which the bomb was. dropped and in a position away from the plane dropping it. This feature avoids the possibility of the bomb following the plane which has-discharged it, for the reason that the latter plane is at a greater altitude and the glider bomb itself cannot reverse its direction so as to fly upwardly.

Referring now to the drawings for a more de-' tailed description of the present invention, and more particularly to Fig. 1 thereof, there is illustrated a directional microphone H) which may phase induction motor, I1 whose stator poles l8 comprise the one'phase while stator poles 18 comprise the secondphase. c

If it be desiredto mountthe structure 0! Fig. 1 upon an aircraft so as to at alltimes inform the pilot of. the direction .01 a source of sound in which he lsiinterested. the directional icrce phone ill will be mounted upon a suitable shaft I 2n whichin turn has splined thereto a gear 2| meshin with a gear 22' carriedby shaft 23/ at j I one end thereof, theshaft being provided atits, =other-endwitha suitable gear 24 for meshing with adriven gear 25 which isoperated from the rotor 26 of the two-phase inductionmotor lfl.

- Mountedabove the directional microphone l0,

and in close proximity thereto lsanon-dlrec 'tional microphone 21, which may be or the crystal type, having its output connected by way or a I 5 suitable; flexible cable'28 to the inputot a gon I 'ventlonal and suitable vacuumtube amplifier 29 whose output in turn is connected by way of leads I c 30 to the second phase 19, l9-oithe two-phase inductionmotorll. By virtue of the fact and the non-directionalmicrophone 21 is or the crystaltype, for example, the outputs ot bqth p microphones due to their inherent operating characteristics, :aresubstantially v out ot phase and since both amplifiers l5 and 29 atop! a slm ilar type, thetwo inputs tothe induction motor I! in phases :8, l8 and l9, 19 are 90 apart which provides the desired condition' for maximum torque of the induction motor l1.

For a better understanding of the output characteristics of the directional microphone l0, reference is now made to Fig. 3, 0f the drawings wherein the form 01' the microphone output is shown in its relation to the angular position oi. the microphone with respect to the sound source. The voltage output of the microphone throughout a rotation of takes the form ofa circle A' while at a reversal'and a rotation of 180 bythe microphone and its ribbon II in an opposite direction thevoltage output takes the form of a second circle B, the two taken. together forming a familiar figure eight pattern. It may be noted further that 180 ambiguity in indication will be prevented for the reason that for each 180' rotation of the directional-microphone a phase reversal of 180 ensues in the output so that the rotation 01 the induction motor is reversedvv for each 180.

When the plane of the ribbon I! of the directional microphone I0 is parallel to the direction of the sound source, the sound waves act equally upon both sides of the ribbon l2 so that the latter does not vibrate and thus there is no output voltage in leads H from the microphone. Even though the output voltage or non-directional microphone 21 is uniform so that at all times phase l9, IQ of motor I! is energized, phase", I8 is de-energlzed and rotor 26 will not move.

As soon, however, as microphone I0 is displaced angularly with respect to the sound source. the plane of ribbon I2 is no longer parallel to the Microphone ill, in the present instance, is-shownas a. ribbon I that directional microp phone In is a ribbon or velocity type microphone,

" am the ource but is is now caused to turn, drivinggear ziwhich inturndrlves gears 22 an'd Z-t mounted" on shaft 23 to angularly move gear 2| and shait 20 therewith until. microphone I is moved angularlyto-a position'wherein its ribbon- I: is again placed in a relationship parallel to the sound waves emanating from the source; At this point the output of microphone no is zero since it is in its null position and phase l8, I8 is de-energized, stopping rotation of rotor 26 and thus'maintaining' microphone- I0 'in its null position. Shaft 20' may be provided with a suitable pointer 3|, to cooperate with'a stationary dial 32 carried by the cr'aft so that at all times pointer3l will indicate the direction of the sound source, I

The same resultas obtained with the use of the structure of'Fig. 1 for indicating the source of sound may be obtainedby a slightly modified form of apparatus which comprisesfas shown in Fig. 2, and which constitutes the subject matter claimed in divisional application .Serial No.'

422,910, filed Dec'ember'13, 194;, three directional microphones '33, 3.4 and 35, eac h, of which is similar to the ribbon typedirectionaLmicrophone adiacentfeach other, in a cluster and insuch-a manner that theribbon of each is displaced 120 with respect to the other. In this arrangement the microphones are stationaryon the craft-andtheir output constitutes three difiere'nt voltages, each depending upon the angular relation of its respective ribbon with the sound source,-.and the'se outputsare in turn communicated by way of cables 36, 31' and 38 to each'.of three phases 39,40; and ti golf an inductive device generallyv designated at 1 With a change in the heading of the craft carrying the novel apparatus 01' Fig. 2, the output of each of the threemicrophones33, 34 and 35 varies in accordance with the-angular displacement of the respective microphones with respect to the sound source and this change in voltage in being communicated to the three phases of the inductive device 42 producesor sets up a magnetic field whose resultant changes .tothereby induce a voltage in a coil 43' carried;by:a rotor 43 of the inductivedevice 42. The voltage thus amplifier 45 to energize; phase 45 0f --a two-phase induction motor 4'! which in all respectsis similar to the induction motor ll of Fig. 1.

Arranged either above .or belowl the .cluster of microphones 33, 34 and 35 and in close proximity thereto, is a non-directional microphone 48, simi lar to non-directional microphonefl of Fig. 1, whose uniform output is, communicated by way of cable 49 tothe input oia-suitable vacuumetube amplifier 50, similarto-amplifier 45,-the'output i of the in duction motor 41-.

Induction motor 41, upon proper energization' of both of its phases, drives its' rotor 52 which'is coupled by suitable means to driving gear '53, meshingwith a gea1t-54a The latter of. which constantly energizesthe second'phas'e' ried by a suitable shaft 55 which at one end is connected with the rotor 44 of inductive device 42 and at itsother' end carries a pointer 56 cooperating with a stationary scale 51. If it be assumed that the craft upon which the novel apparatus oi Fig. 2- is mounted is in such a posltion that the outputs of the microphones 33,36 and 35 are of a value whereby the rotor Midi; inductive device is maintained in a position shown in Fig. 2' and subsequently the craft changes its heading withfrespect to the sound source; the relationship of the microphones changes with respect tothesource and this change produces three different voltages in cables 35,

31' and 38, whose ratios depend upon the amount of angular displacement from the direction of the sound source, thus producing a different energization of each of the three phases of the inductive device '52 whereby the magnetic fields 3 created by each of the phases "vary, thus having a different resultant due to which a voltage is induced in coil 43 of rotor 44; This voltage is amplifled by way of amplifier 45 to energize phase 46 I of induction motor 41. 'Sincethe second phase 5| of the induction motor is already energized due to the output of non-directional microphone 48-, rotor 52 of the induction motor is caused to move angularly thereby driving gear 54 by way of gear 53 to angularly move rotor 44 to a new Ill'of'Fig. 1. These microphones 33, 34 and are arranged or mounted one "above the other,closely position in which the voltage within coil 53 is reduced to zero, This is the null position for the rotor 34 and phase '46 of the induction motor is de-energized and atthe same time that rotor M or the inductive device 42 has been turned to its null position pointer 56 hasb'een moved relatively with respect to dial or scale 51 to thus indicate the V ing glider type missile for use against night-fiyinduced in coil 43 is amplified in a. vacuum .tube

direction of the source of sound.

It will now be apparent to those skilled in the art that the apparatus of Fig. 2 is adapted for continuously indicatingthe' sound source and differs from the structure of Fig. l in that in the latter, microphone i0 is always urged bodily in the direction of, the sound source whereas in the structureof Fig. 2 the microphones .are all mounted 'Stationarily with respect to the craft.

The above-described apparatus together with its principle of operation is adapted for many uses,- one of which is of great importance in that it may be applied to a self-directed sound seeking enemy aircraft. This latter use is diagrammatically illustrated in Fig. 4 wherein is shown a glider 58 having wings 59 and elevator and rudder control surfaces 60 and 6|. Mounted in the nose oi the glider are two directional microphones 62 and 53 arranged one above the other and displaced angularly with respect toeach other so that their ribbons are substantially 90 apart, "the output of each being fed into the input of '-jtwo suitable vacuum tube amplifiers 64 and 55.

Likewise mounted in the nose of the glider 59 and either above or below, and closely adjacent the microphones 62 and 53, is a non-directional microphone 36, whose output in response to the sound waves emanating from a source of sound amplifiers is"- out of phase'with the outputs of microphones 52 and 63 and is, fed into the input of a suitable-"amplifier 67, similar in all respects to 64 and. 65 of the directional microph nes.- i

The elevator control surfaces 60 are Joined by a common shaft 58 having mounted thereon a gear sector 63 which meshes with a worm l0 drivenfby' the shaft 1| of aninduction motor 12.

This induction motor is a'two-phase induction motor similar in all respects to those illustrated.

in both Figures 1 and 2.

The rudder surface 6| is mounted for movement with a suitable -shaft"|3 carrying a gear sector 14 meshing with a gear l5 driven by shaft 16 of a second induction motor 11 constituting a two-phase inductionmotor of the same type as motor I2.

One phase of each of theinduction motors 12 and T! is energized by ways of leads 18 from the amplifier 6'! of the non-directional microphone output. The second phase of induction motor 12 is connected by way of -lead I9 with the ampli fler 65 of the output of the directional microphone 62 which may be termed in this instance the elevator control microphone, while the second phase of the induction motor 11 is connected byway of lead 80 with the output of amplifier 64 connected to the output of the directional microphone 63 which may be termed the rudder control micrbphone; Each of the amplifiers 64, 65 and 81 may be connected] with 1a suitable source ofpower8l'. g;

For abetter understanding of the operation of the novel device of Figure 4, let it be assumed that a defensive plane is flying over an enemy bomber which may be invisible due to either fog or night flying. The .defensive plane discharges glider 58 and the latter will begin to glide and descend while the defensive plane mayso into a slight climb to avoid energizing the controls at.

thenose of the glider. Since th glider has no motive power of its own, it is incapable of climb ing and will thus tend to glide, or under the effect of gravity be pulled earthwardly. Initially, the accuracy of the glider is low, but as it travels downwardly the volume of the sound waves from the source increases and th directive accuracy of the glider increases for the reason that microa predetermined height above the earth the bomb may be discharged, thereby protecting forces.

There is thus provided a novel apparatus adaptthe ground ved to either indicate the source of sound-waves .or to control and automatically direct a device phones 62, 63 and 66 at the nose of the glider become energized, in the manner hereinabove set forth in connection with Figures 1 and 2, so that in accordance with the angular relation of the directional microphones 62 and 63 with respect to the source of sound, the second phases of the induction motors l2 and T! will become energized so as to operate both rudder and elevator surfaces 60 and GI to change the direction ofilight of the'glider 58 until the latter is flying toward its target or the source of sound. Even if the target changes its heading, the glider willnevertheless follow it fo the reason that the change in heading changes the sound source thus changing the outputs of directional microphones 62 and 53 which in turn causes a variation in the operation ofinduetion motors 12 and 71 to again operate the rudder and elevator surfaces to direct the glider after the target.

From the foregoing it will now be apparent to those skilled in the art that a novel automatic sound directed device has been provided which I may be termed a target seeking dirigible bomb of the sonic type which is particularly adapted for use against night-flying enemy aircraft. The projectile may befurther provided with a suitable relay 82 which is connected to the output of amplifier 6! of the non-directional microphone 66 and may constitute a marginal relay which will only respond to a predetermined amount of volume in amplifier 61 which may be provided with a suitable well-known automatic volume control device. This feature is useful in that the target seeking dirigible bomb need not actually strik or hit its target but may be detonated due to the energization offi elay 82 when the target seeking bomb is within a predetermined distanceof the target whichdistance such as a target seeking dirigible bomb toward the sound source which consists in the conversion of sound waves into electrical power which may or may not be amplified so as to driveindieating or control means as afunction of the direction of the sound source.

Although three embodiments of the-invention have been illustrated and described in detail, it

is to be expressly understood that the invention is not limited thereto. For example, various types of directional microphones besides the ribbon type microphone hereinabove described may be used and the non-directional microphone may be other than the crystal type discussed. The directional microphones together with the non-directional microphone must be arranged one above the other and in close proximity to each other so that the sound waves intercepted .by each are of a like phase so as to assure proper operation of the indicating or control means. Various changes may also be made in the design and arran ment of the parts without departing from the fiirit and scope of the invention as the same will now be understood by'those skilled in the art. For a definition of the limits of the invention reference will be had primarily to the appended claims.

What is claimed is:

1. In combination, a plurality of directional microphones arranged in a predetermined manner and a non-directional microphone, therespective outputs of said directional microphones being a function of the angular displacement, of I said microphones with respect to a sound source, said non-directional microphone having a substantially continuous energy output, driven means, means energized in accordance with the outputs of said microphones to drive said driven means, and means operated by said energized means to counteract the torque energized means. I

2. In a system for indicating the direction of a sound source, a rotatably mounted directional microphone normally maintained in a predetermined position with respect to said source so as produced by said to have a zero voltage output in such position,

and having a varying voltage output as'a function of its angular displacement from said predetermined position relative to said sound source, a non-directional microphone having a substantial ly uniform voltage output in response to sound waves emanating from said source, driving means connected directly with the output of said microphones operative during a displacement of said directional microphone from saidpredetermined position to drive said .directional microphone to said predetermined position whereby said driving means are deenergized, and means driven by said driving means for indicating the direction of said sound source.

o 3. Inga system tor indicating the directionof T a sound source, a :rotatably mounted directional microphone normally-maintained in apredcter- :o mined position with respect to' said source so as to have a zero voltage output insuch position and having a varying voltage outputas a function of its angular displacement fromsaid predetermined position relative to said sound source,- a non-directional microphone having a: substantially uniform voltage output in response to sound waves emanating from said source, amplifiers for oi the relative" position of each of said microeach of said outputs, a polyphase induction mo-- tor connected to said amplifiers and having its phases energized by the amplified outputs of said microphones whereby, upon a relative displacement 01' said directional microphone with'said sound source, said directional microphone is drivon to its normally predetermined position and the motor is. deenergizem and means driven i by said motor for indicating the direction of said sound source. 4. In a system for indicating the direction of a sound source, a plurality of directional microphones arranged in a predetermined manner so that their'respective outputs vary as a function of the relative position oi. each of said microphones relative to said sound source, a non-directional microphone having a substantially uniform energy output in response to sound waves emanating from said source, means for indicating the direction of said sound source, means associated with the output of said non-directional and directional microphones operative during a relative displacement of said directional microphones and said sound source for operatingsaid indicating means, and means operated}. simultaneously with said indicating'means ion'counteracting the driving torque produced by said operating means.

5. In a system for indicating the direction of a sound source,, a. plurality of directional microphones arranged in a predetermined manner so that their respective outputs vary as a function of the relative position of each of said microphones relative to said sound source, a non-directional microphone having a substantially uni- ,form energy output in response to sound waves emanating from said source, means for indicating the direction of said sound source, a polyphase induction motor having its, 'phases energized by the. outputs of said non-directional and directional microphones and operative during a relative displacement of said directional microphones and said sound source for operating said indicating means, and means operated simultaneously with said indicating means for ounteracting the driving torque produced by said motor.

8. In a system for indicating the direction of a sound source, a plurality of directional microphones arranged in a predetermined manner so that their respective outputs vary as a function of the relative position of each of said microphones relative to said sound source, a non-directional microphone having a substantially uniform energy output in response to sound waves emanating from said source, an inductive device having stator windings energized by the outputs of said directional microphones and a rotor wind ing having a voltage induced therein in response to a relative displacement of said directional microphones and said sound source, means for indicating the direction of said sound source, and a polyphase induction motor having certain of its phases energized by the output 01 said non-directional microphone and other of its phases energized by the voltage induced in the rotor winding of said inductive device during said displacephones relative to said sound source, a non-directio'nal microphone having a substantially uniform energized by the output of said non-directional microphone and the other phase energized by the voltage induced in the rotor winding of said inductive device during said displacement of the directional microphones and the sound source for simultaneously operating said indicating means and said rotor winding whereby the. driving torque produced by said motor is counteracted.

8. In a system for indicating'the direction of a sound source,a plurality of directional microphones arranged in a predetermined manner so that their respective outputs vary as a function of the relative position of each of said microphones relative to said sound source, a non-di. rectional microphone having a substantially uni-'- form energy output in response to sound waves emanating from said source, an inductive device having stator windings energized by the outputs oi said directional microphones and a rotor winding having a voltage induced therein in response .to a relative displacement of said directional microphone and the other phase energized by the voltage induced in the rotor winding of said inductive device-during said displacement oi the directional microphones and the sound source for operating said indicating means and said ro= tor winding whereby the latter attains a null po-' sition to tie-energize said other phase of said in duction motor. a

9. Ian system for determining the direction of a sound source, a two-phase induction motor, a

rotatably mounted directional microphone nonmally maintained in a predetermined null posi-- tion with respect to said source so asto' have a zero voltage output in such null position and having a varying voltage output as a function of its angular displacement from said null position relative to said sound source, one phase of said motor being directly'connected for energization with the-output voltage of said directional microphone. a non-directional microphone having a substantially uniform voltage output in response to sound waves emanating from said source, the other phase of said motor being di rectly connected for constant energization bythe output voltage of said non-directional microphone, and connecting means actuated by said motor upon a relative displacement of said di...

'rectional microphone from said null position relative to said source for driving said directional microphone to its null position and de-energizing said one phase of said motor. v

10. In combination, directional microphone means the output of which is a function of the angular displacement of the microphone means with respect to sound waves emanating from a sound source, a non-directional microphone having a substantially continuous energy output in response to sound waves transmitted from said source, an indicator for indicating the direction of the sound source, a polyphase induction motor having one of its phases energized by the direc rtional microphone means and another of its phases energized by the non-directional microphone, whereby a driving torque is produced by the motor in proportion to the directioncf the sound source with respect to the position of the directional microphone means, and means driven by said motor to counteract the driving torque produced by said motor and to simultaneously move the indicator to a position corresponding to the direction of the source of sound.

11. In combination, directional microphone means the output of which is a function of the angulardisplacement of the microphone means.

with respect to sound waves emanating from a sound source, a non-directional microphone having a substantially continuous energy output in response to sound waves transmitted from said source, a polyphase induction motor having one of its phases energized by the directional micro- I phone means and another of its phases energized by the non-directional microphone, whereby a driving torque is produced by themotor in proportion to the direction of the sound source with respect to the position of the'directional microphone means, means driven by said motor to counteract the driving torque produced by said 'motor, and means associated with said driven means adapted to move simultaneously with said driven means to a position corresponding to the direction of the source of sound.

ALFRED A. STUART, JR.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,506,552 Angus Aug. 26, 1924 1,842,342 Eaton Jan. 19, 1932 1,842,345 Eaton Jan. 19, 1932 2,138,966 Hafner Dec. 6, 1938 FOREIGN PATENTS Number Country Date 344,967 Great Britain Mar. 19, 1931 469,322 Great Britain July 22, 1937 Great Britain Apr. 1, 1938 

